1. Field of Invention
This invention relates to a new and improved high voltage direct current (HVDC) power transmission system employing a novel floating current order control subsystem.
More specifically, the invention relates to such an HVDC system wherein a novel floating current order control subsystem is included for developing a continuously available floating current order control signal which is representative of the magnitude of the direct current flowing in the HVDC link at a given time plus or minus a predetermined current margin and which goes up or down in magnitude value with changes in the HVDC system direct current magnitude. This floating current order control signal is provided as one of the operation-regulating input control signals to the regulator of the power converter of the HVDC system which is not in current control at the time. There are at least two power converters in an HVDC system, one at each end of the HDVC power link, and each power converter may be provided with a floating current order control subsystem although only one is effective at any given time to effect nearly "bumpless" transfer of control of current from one power converter to the other should such transfer of current control be required during operation of the system.
2. Prior Art Problem
During the start-up and shut-down, and even under normal running of an HVDC system, it is anticipated that certain transient conditions can occur such as rectifier and/or inverter voltage interruptions in the form of voltage dips or loss. The occurrence of such transient conditions may result in a mode switching change in the control of the regulator of the power converter subjected to such transient condition. Such operating mode switching changes are described in detail in U.S. Pat. No. 3,832,620 issued Aug. 27, 1974 entitled "Regulating Mode Selector Scheme for an Electric Power Converter" by Ernest M. Pollard, assigned to the General Electric Company. Mode changes usually involve a shift in the control of the current flowing in the HVDC link from one power converter to the other. In known HVDC systems, a communications link is relied upon to communicate the current order in an attempt to achieve as close to a "bumpless" shift as possible, (i.e. a shift which is not accompanied by a substantial change in direct current magnitude if a mode change occurs.) In the event there is a loss in the communications link (due for example to a storm) then the need to change current order may not be communicated to the opposite power converter. In such eventuality, complications can develop in the operation of the HVDC system possibly accompanied by an unintentional reversal in power flow through the system.
To avoid such complications in the absence of communications and still allow current order changes as long as the end where the change originates remains in current control, the present invention was developed. Normally, the end where the current order change originates is designed to be in current control. It only loses current control during abnormal conditions which are usually temporary in nature. Therefore, for all practical purposes, the ability to change power transfer is maintained even in the absence of communications through use of the present invention.